This invention relates to a device for measuring thermal conductivity of liquids under heating by impulsive light from a laser (laser flash), which is especially suitable for measuring thermal conductivity of hot melts.
In the conventional laser flash heating type measuring device of this class, a sample liquid, for instance, mercury is sealed in a container of boron nitride with a lid of transparent quartz plate and irradiated from above by a laser flash, while measuring the response in temperature rise at the bottom surface of the sample layer by a thermocouple which has its detecting end at the bottom of the container.
However, such conventional measuring device has the following problems.
(1) The measurement is possible only with an opaque liquid like mercury which is impermeable to laser light and which can receive the energy of the irradiated laser flash at its surface, and not with most of other liquids which are permeable to the laser light.
(2) Where a liquid of good thermal conductivity like a liquid metal is to be measured, it is easy to choose a container material with a thermal conductivity low enough as compared with that of the sample liquid. However, many of other liquids do not have much difference in thermal conductivity from the container which holds the sample, making it difficult to obtain the heat conductivity and thermal diffusivity of a sample from the measurement of its temperature response.
(3) It is extremely difficult to seal up the sample liquid in the container and a thin gas absorption layer is apt to be formed between the sample liquid and the wall surfaces of the container, producing a contact resistance which causes unignorable errors to the measured values.
(4) Strick assessment of the thickness of the sample liquid layer is required in analyzing the measured temperature response, so that it is necessary to known precisely the thermal expansion coefficient of the container for measurements at high temperatures. In addition, there is a possibility of the sample liquid overflowing from the container or of a void space being formed within the container when the measuring temperature is changed, due to the difference in thermal expansion coefficient between the container material and the sample liquid.
As for the conventional means for measuring thermal conductivity of hot melts, there have thus far been proposed a stationary means, a non-stationary hot-wire means and a non-stationary plate heating means.
The stationary means requires a long time for creating the stationary state and a high degree of expertness, with unavoidable heat losses to the ambient which cause greater errors to the measurement as the temperature is increased.
The non-stationary hot-wire means is considered to be reliable at temperatures below 500 K but its measuring device has a shape which cannot suppress the convection in the sample liquid and thus is unsuitable for the measurements at high temperatures.
The non-stationary plate heating means employs a horizontal plate which is immersed in a hot melt and to which current is applied stepwise or at a predetermined cycle to generate Joule heat. The curve of the temperature rise of the liquid is measured at a predetermined distance from the plate or the phase difference between the periodical variation of the liquid temperature and the cycle of the current flow through the plate is measured. In this case, it is also difficult to suppress the convection, coupled with a problem that the application of Joule heating to electrically conductive liquids by electric current necessitates a special contrivance for the insulation of the heating plate.